Help needed with P0300 and P0302 codes on 2004 TJ 4.0

[DSMITH]

Hey folks. Have a 2003 TJ with a 4.0 and 105,000 miles. Over the past few weeks it would "randomly" have rough idle at start if the engine was warm - very random happening maybe once a week and lasting only 5-10 seconds. Thought maybe I got some bad gas. Today check engine light popped on and gave error codes P0300 and P0302 with a very rough idle while driving. Parked it, and restarted roughly 10 minutes later with a rough idle - within a mile rough idle went away. Ran a diagnostic and got random, inconsistent results ( 2 and 3 misfire, then a 4 and 6 misfire, then a 1 misfire ). Diagnostic would not even consistently show the misfires. Reset check engine light and no issues since. Spark plugs are new Champions, and coil pack seems to be fine. Could this be a crank position sensor? Where do I trouble shoot first?

 

[TJ Hunnicutt]

I am not personally a big fan of Champion's plugs as I have had just this sort of issue with them in the past. But then, that was back in the day before platinum or iridium plugs. Check out this thread which hits on the heat soak issue as well, which also may be your problem.

 

[DSMITH]

Thanks TJ Hunnicutt. I've read about the heat soak issue, and with temps in high 90s and heat indexes over 104 the past few days, I thought this could be the issue. I've ordered the shield just in case, but it looks like I should get only an error code for #3 if this was the only issue. These codes are inconsistent and bouncing all over the place, and I think cylinder 3 was the only one not to show during diagnostic test. Champion 7034 were installed recently, and I've run them in my 05 Rubicon for years, so don't think that's the issue in this 03 Sport.

 

[AjRagno]

It’s heat soak. There’s a TSB that includes the 2004 TJ.

https://wranglertjforum.com/threads...idle-after-restart-following-a-hot-soak.2345/
https://www.amazon.com/gp/product/B007O3QHDK/?tag=wranglerorg-20

There is also a thermal insulation kit available for the fuel rail and injectors. I just finished installing this on my 2005 LJ.

https://www.amazon.com/gp/product/B015NOKWQY/?tag=wranglerorg-20

 

[TJ Hunnicutt]

Heat soak can cause issues beyond cylinder #3. Especially if you have no heat shield at all, the entire fuel rail can become very hot and cause vapor lock on any and/or all of the cylinders and, of course, much more so in hot weather.
Put the heat shield and insulation on, then go from there.

 

[DSMITH]

Thanks to both of you. DEI heat shield kit ordered. I'll give it a shot - seems like one of the cheapest starting points. Not much heat shield left from the factory piece, and what is there is basically destroyed.
Is there any chance it could be the crank position sensor? Anyone with CPS issues in the past that can share symptoms?

 

[twistedlv]

Lots of things can lead to general misfire conditions. The Factory Service Manual has very good troubleshooting guides. Get yours Here.

 

[CharlesHS]

I already done the usual spark plugs, injector insulators and intake shield.
Another thing to consider is the heat may be causing pre ignition in the combustion chambers...
I have used SeaFoam to clean the combustion chambers several times since I purchased the Jeep which has helped quite a lot.

 

[pagrey]

If that doesn't work try the factory NGK 3459 (ZFR5N) plugs. They have two spark gaps and are much more resistant to miss-fire when conditions aren't ideal. Although the other plugs are called twin I think that just refers to the platinum or iridium on both ends. The NGKs aren't expensive.

 

[CharlesHS]

If that doesn't work try the factory NGK 3459 (ZFR5N) plugs. They have two spark gaps and are much more resistant to miss-fire when conditions aren't ideal. Although the other plugs are called twin I think that just refers to the platinum or iridium on both ends. The NGKs aren't expensive.

After trying the platinum tips recommended on the forum and receiving numerous misfire codes; I replaced them with the NGK ZFR5N spark plugs recommended by my friend who was a Factory trained Jeep technician for 30 years. The Jeep has run quite well since then....

 

[DSMITH]

Thanks to everyone for their responses, but plugs keep coming up. Champion 7034 plugs should be fine in this engine, and they are gapped correctly. If plugs were the issue, why would the rough idle only occur on a warm start? The more I read ( thanks TJ Hunnicutt and AJRagno ) the more it seems like the heat soak fits the issue. Seems like plugs would throw codes any time and not smooth out after a few blocks.

 

[CharlesHS]

Thanks to everyone for their responses, but plugs keep coming up. Champion 7034 plugs should be fine in this engine, and they are gapped correctly. If plugs were the issue, why would the rough idle only occur on a warm start? The more I read ( thanks TJ Hunnicutt and AJRagno ) the more it seems like the heat soak fits the issue. Seems like plugs would throw codes any time and not smooth out after a few blocks.

I thoroughly agree that the heat soak problem is primarily caused by heat coming up from the exhaust manifold and cooking the fuel injectors; especially #3.
That was my first project to replace and insulate the fuel injectors which removed the rough idle upon restarting after 5-10 minutes.
The sparkplugs was an after thought that resolved the remainder of the random misfires.

 

[Jason from SoCal]

Just stumbled across this thread.... Sounds like you're well on your way to finding your solution.

I too had a P300 mystery on my hands. I never really noticed an explicitly rough idle or any drivability issues, but the check engine like would consistently come on if I exceeded 3000 rpm. I changed/checked everything. CPS, Fuel Rail Pressure, plugs, coils... nothing fixed it.

The solution to my problem was a simple reset of the engine computer to have it recalibrate the crank position sensor. CEL light hasn't been on since. Immediately after the reset the engine had some strange idle behavior. But after a few drive cycles it is running great. Supposedly it takes 50 drive cycles for the computer to fully recalibrate all the ECU functions. You can find the reset procedure online, it involved disconnecting the battery and cycling the headlights. If it wasn't for the fact that it solved my extremely annoying problem, I would have thought someone was pulling my leg.

 

[Rubicon88]

Plugs..age?
Wires..age?

Fuel pressure how much?

I'd start with the fuel first.

When you first start your jeep it has a higher fuel air ratio...in theory...to get it warmed up.

Once warmed it will correct the mixture(your rough idle going away)

My guess is your fuel pressure/filter

Good luck

 

[RMETeeJay]

Just stumbled across this thread.... Sounds like you're well on your way to finding your solution.

I too had a P300 mystery on my hands. I never really noticed an explicitly rough idle or any drivability issues, but the check engine like would consistently come on if I exceeded 3000 rpm. I changed/checked everything. CPS, Fuel Rail Pressure, plugs, coils... nothing fixed it.

The solution to my problem was a simple reset of the engine computer to have it recalibrate the crank position sensor. CEL light hasn't been on since. Immediately after the reset the engine had some strange idle behavior. But after a few drive cycles it is running great. Supposedly it takes 50 drive cycles for the computer to fully recalibrate all the ECU functions. You can find the reset procedure online, it involved disconnecting the battery and cycling the headlights. If it wasn't for the fact that it solved my extremely annoying problem, I would have thought someone was pulling my leg.

X2, I had a bad OPDA and would still throw these codes even though I had replaced it and knew everything else was good. A reset made them go away.